1. Field of the Invention
This invention relates to circular knitting machines and in particular the yarn feed to such machines for producing tubular knitted articles such as stockings, and provides a device for presenting the various yarns for forming said articles to the working needle faces in a double-cylinder machine.
2. Description of the Related Art
In general, double-cylinder circular knitted machines consist essentially of two rotating cylinders with tricks in their outer cylindrical surface. The tricks represent the guides for the needles which during their vertical travel form the stitch loops in cooperation with the sinkers.
The two cylinders are positioned one above the other with their two ends facing so that the tricks of the two cylinders correspond axially, in order that the needle of each trick can switch between the two cylinders or faces and engage alternately with one and the other of two identical sliders which slide in the two facing tricks. The two sliders operate alternately, in the sense that a slider operates on receiving the needle from the opposite face and does not operate after consigning the needle to the slider of the other face, which is operating.
The basic stitch forming process will now be described with reference to FIGS. 1 and 2.
The two cylinders are indicated by 1A and 1B and their tricks by 2. The number of tricks is equal to the number of needles 3 which slide within them with vertical reciprocating motion, switching between the two cylinders. For stocking production each cylinder comprises up to about 400 tricks.
In double-cylinder machines the needles are of a special type to be able to switch between the two faces, and are in the form of double-hook needles, with each of which there correspond two sliders disposed in the facing grooves and driven by butts which engage raising and lowering cams controlling the vertical reciprocating motion of the sliders and needles associated with them. A double-hooked needle and slider unit and its operation are described in the European Patent Appln. Public. No. 0 428 205 of the present Applicant.
The needles 3 rotate with the cylinders to operate with reciprocating movement between a minimum travel and a maximum travel position within their cylinder, with the possibility of intermediate positions, by the action of a series of raising cams 4A/B and lowering cams 5A/B, the contours of which engage the butt of the slider 6A/B, causing it to move axially upwards and downwards. The needles cooperate with sinkers 7 arranged in a ring between one needle and the next to form the stitch loops, which when connected together form the tubular fabric. Generally the sinkers 7 undergo small movements to regulate the knit, i.e. the length of yarn between the needle and sinker, which determines the size of the stitch loops.
The cylinders 1 are rotated and with them there rotate the needles 3 and sinkers 7.
During their vertical reciprocating movement the needles are fed in fixed angular positions and at the most advanced levels of the travel within their cylinder 1 by feed stations which, when required, present the needles with the yarn to be knitted into the knitwork at that knitting course and in that angular position. At each feed change the previously fed yarn has to be changed over with the yarn forming the new feed. FIGS. 2a and 2b show the method of changing the yarn in a conventional feed station. Each feed yarn is carried by a yarn guide 8 which withdraws the yarn from a bobbin 9 via a series of deviators, of which only the deviator 10 located on the shaft of the yarn guide 8 is shown. With the assembly of yarn guides 8 there is associated an assembly 11 of grippers for cutting and controlling the ends of the feed yarns. The number of grippers is equal to the number of yarn guides, four in the illustrated embodiment (8a . . . 8D which withdraw yarn from the respective bobbin 9a . . . 9d), each operating on the feed yarn of one yarn guide and cooperating synchronously with it.
The various yarn guides 8 are positioned at different levels and/or radial distances so that the trajectories t through which they travel do not interfere with each other and so that any one yarn guide can convey its own yarn into operation without it preventing another yarn guide from conveying its yarn out of operation.
The yarn guides are generally driven to extend and retract along their trajectories t to convey the various feed yarns into and out of operation by systems of known type, consisting for example of rotating cams which operate reciprocating levers. The rotation of said cams is generally determined by pneumatic or magnetic controls which engage the cams at the appropriate time with the needle cylinder drive system, to derive their motion from it. A magnetic control system independent of the yarn guide movement is the subject of the copending Italian patent application No. 21040 A/90 in the name of the present applicant.
Each gripper consists of a hook 12 contained between a fixed blade 13 and a part 14 which rises and descends relative to these two. The blade 13 has a cutting edge and cuts the yarn 15 on the fabric side, the yarn making contact with it when the hook 12 which has gripped the yarn is lowered. The part 14 has bevelled edges and grips the residual yarn end on the bobbin side, so that this yarn end is held by the gripper 11 and yarn guide 8 in a defined position.
The conventional yarn changing process using the device of FIGS. 2a and 2b is as follows:
the non-operating yarn, shown by full lines, is retained by the gripper hook 12aand by the yarn guide 8a in position A, as shown in FIG. 2b for the yarn guides 8b, 8c and 8d. The machine is operating with other yarn, PA1 the yarn has now to be brought into operation. The yarn guide 8a is moved gradually towards position B along the trajectory t, the yarn assuming the configuration shown by dashed and dotted lines, PA1 the yarn is transferred into operation. When the yarn guide is in position B the yarn 15 intersects the circular path 16 of the needles 3 and is gripped by the needles 3B, which rise in that angular position following the final position B. A very short time (for example 5-10 needle steps) after the yarn guide 8a has reached position B, the hook 12a corresponding to the yarn guide 8a rises to release the yarn end retained against the bevelled part 14 and is then lowered so as not to hinder other yarns which are to be moved into the non-operating position. The yarn guide 8a remains in position B for the entire time during which this yarn is to be fed to the needles, the yarn 15, shown by continuous lines, being dragged into operation by the needles and being unwound from the bobbin 9a via the deviators and the yarn guide ring 17, PA1 the yarn is now taken out of operation. The yarn guide 8a is retracted from position B to position A and the hook 12a raised to its maximum level to interfere with the yarn which is taut between the needles in position 3A and the yarn guide in position A, as shown in FIG. 2b. A very short time (for example 5-10 needle steps) after the yarn guide has reached position A, the hook 12a is lowered and cuts the yarn with the cutter 13, leaving the downstream end attached to the fabric and retaining the upstream end gripped between the lowered hook 12 and the bevelled part 14. The commands for each yarn guide 8 and the corresponding hook 12 are provided by cam devices which provide the necessary synchronization between the hook and the yarn guide, which operate as a pair. The conventional device described up to this point has considerable drawbacks, of which the following should be mentioned.
The hook 12 which rises in phase with the release of the end of the yarn which is entering into operation can interfere with another yarn which is being taken out of operation and which could be gripped in a position which is not its own, so mixing the pairs of yarn guides and hooks and losing the necessary synchronization between the two members. With high-speed machines and frequent yarn changes while working, this problem could result in whole batches of defective stockings, unless the speed is reduced, with resultant reduction of machine productivity.
If two yarn guides are required to take their yarn simultaneously out of operation, the two yarns would both be transferred to the first raised gripper, hence producing the aforesaid defect. Their take-out from operation must be done at different times, thus imposing textile limitations on the machine.
Wool or other voluminous yarns soil the device 11 with the fibres which are inevitably lost during cutting with the blade 13 and the clamping with the part 14. The unit is generally equipped for cleaning by air blasts at every opening at maximum level, but this is not sufficient and the unit 11 has to be periodically dismantled completely and cleaned. The cutting edge of the blade 13 also requires frequent sharpening. The yarn end left upstream on the fabric is very long and has to be cut off during subsequent finishing of the product stocking.
If operating with two yarns of different count in the same yarn guide, for example in "vanise" production, the thicker yarn is gripped whereas the thinner yarn can escape the gripping. This loss of the yarn end means that the stocking is inevitably rejected during quality control.